1. Field of the Invention
The invention relates generally to electronic current sources, and more particularly to a current source suitable for use as a loop filter in a phase-locked loop.
2. Description of Related Art
Without limiting the scope of the invention, this background information is provided in the context of a specific problem to which the invention has application. Phase-locked loops, a.k.a. PLLs are ubiquitous, found in many applications ranging from automobiles to microprocessors. A PLL is commonly used to synthesize a high frequency clock signal from and synchronous to, a lower frequency reference signal. In its most simplest form, a PLL comprises a voltage controlled oscillator (VCO), a prescaler, a phase comparator, and a loop filter. The output of the phase comparator drives the loop filter which typically is a low pass filter with gain. The loop filter provides an output voltage which drives the input of the VCO. The VCO output is then scaled through the prescaler and fed back to one input on the phase comparator. A second input on the phase comparator is coupled to a reference signal that the VCO output frequency attempts to track, scaled by an mount set by the prescaler. The phase comparator generates an error signal to force the frequency of the VCO up or down depending on whether the scaled VCO output is leading or lagging the phase of the reference input.
A common trend in modern electronics is the reduction of the power supply voltage. This is being done, for the most part, to reduce power consumption and to increase integrated circuit densities. Although these objectives are laudatory, the attendant drawbacks of lowering the power supply voltage can compete with the benefits obtained. Specifically, as the power supply voltage is reduced in a PLL, the available voltage swing is reduced thereby compressing the frequency-lock range of the PLL. A particular problem in MOSFET circuitry with reducing the power supply voltage is overcoming the current limiting characteristics as the drain to source voltage (V.sub.DS) approaches zero.
Known prior art loop filters for use with VCOs and PLLs include, U.S. Pat. No. 4,922,139, issued May 1, 1990, entitled "Filter Circuit For Generating A VCO Control Voltage Responsive To The Output Signals From A Frequency/Phase Discriminator" which employs a form of gain control and V.sub.DS compensation but has a limited output voltage swing.
Another approach found in U.S. Pat. No. 4,987,387, issued Jan. 22, 1991, entitled "Phase Locked Loop Circuit With Digital Control" is silent on V.sub.DS compensation but discloses an automatic gain control circuit (120) employing an op-amp (121) for frequency gain, undesirable for reduced voltage operation.
Accordingly, it can be seen that there is a need for a current source suitable for use as a loop filter in a PLL which can overcome the obstacles of operating at reduced power supply voltage.